U.S. patent application number 10/143280 was filed with the patent office on 2003-11-13 for multiple membrane structure and method of manufacture.
Invention is credited to Gabriel, Kaigham J., Xie, Huikai, Zhu, Xu.
Application Number | 20030210799 10/143280 |
Document ID | / |
Family ID | 29400086 |
Filed Date | 2003-11-13 |
United States Patent
Application |
20030210799 |
Kind Code |
A1 |
Gabriel, Kaigham J. ; et
al. |
November 13, 2003 |
Multiple membrane structure and method of manufacture
Abstract
A direct digital microphone is constructed of a plurality of
first membranes each formed by a micro-machined mesh supported by a
substrate. Each of the membranes has a first and a second position.
A second membrane is supported by the substrate and positioned
above the plurality of first membranes to form a chamber between
the plurality of first membranes and the second membrane. A
pressure sensor is responsive to pressure in the chamber. Drive
electronics are responsive to the pressure sensor for controlling
the positions of each of the plurality of first membranes. Output
electronics are responsive to the positions of the plurality of
first membranes to produce a digital output signal. A stacked
membrane structure and methods of fabrication and operation are
also disclosed.
Inventors: |
Gabriel, Kaigham J.;
(Pittsburgh, PA) ; Zhu, Xu; (Pittsburgh, PA)
; Xie, Huikai; (Pittsburgh, PA) |
Correspondence
Address: |
THORP REED & ARMSTRONG, LLP
ONE OXFORD CENTRE
301 GRANT STREET, 14TH FLOOR
PITTSBURGH
PA
15219-1425
US
|
Family ID: |
29400086 |
Appl. No.: |
10/143280 |
Filed: |
May 10, 2002 |
Current U.S.
Class: |
381/173 ;
381/171; 381/190; 381/91 |
Current CPC
Class: |
H04R 31/00 20130101;
H04R 25/00 20130101; H04R 3/005 20130101; H04R 1/04 20130101; H04R
19/005 20130101; H04R 1/005 20130101 |
Class at
Publication: |
381/173 ; 381/91;
381/171; 381/190 |
International
Class: |
H04R 001/02; H04R
025/00 |
Claims
What is claimed is:
1. A structure carried on a substrate, comprising: a first membrane
formed of a micro-machined mesh supported by the substrate; and a
second membrane supported by the substrate and positioned above
said first membrane to form a chamber therebetween
2. The structure of claim 1 wherein said first and second membranes
are fabricated one above the other to form an integral
structure.
3. The structure of claim 1 wherein said second membrane is
mechanically connected above said first membrane to form a
composite structure.
4. The structure of claim 3 wherein said second membrane is one of
a fabricated membrane and a cover membrane.
5. The structure of claim 1 wherein said first membrane is
comprised of a first micro-machined mesh and a first material
sealing said mesh and wherein said second membrane is comprised of
a second micro-machined mesh and a second material sealing said
second mesh.
6. The structure of claim 5 wherein said first material and said
second material are the same.
7. A structure carried on a substrate, comprising: a first
micro-machined mesh supported by the substrate; a second
micro-machined mesh supported by the substrate and positioned above
said first mesh; and a material for sealing said first and second
meshes to form first and second membranes, respectively.
8. The structure of claim 7 wherein said first and second membranes
are fabricated one above the other to form an integral
structure.
9. The structure of claim 8 wherein the gaps of said second mesh
are larger than the gaps of said first mesh.
10. The structure of claim 7 wherein said second membrane is
mechanically connected above said first membrane to form a
composite structure.
11. A stacked structure comprising at least two membranes, and
wherein at least one of said membranes is formed of a
micro-machined mesh.
12. The structure of claim 11 wherein said at least two membranes
are fabricated one above the other to form an integral
structure.
13. The structure of claim 11 wherein a top one of said at least
two membranes is mechanically connected to said stack.
14. The structure of claim 13 wherein said top one of said at least
two membranes is one of a fabricated membrane and a cover
membrane.
15. The structure of claim 11 wherein said at least two membranes
are comprised of a first micro-machined mesh and a first material
sealing said mesh and a second micro-machined mesh and a second
material sealing said second mesh.
16. The structure of claim 15 wherein said first material and said
second material are the same.
17. A microphone constructed on a substrate, comprising: a
plurality of first membranes each formed by a micro-machined mesh
supported by the substrate, each of said membranes having a first
and a second position; a second membrane supported by the substrate
and positioned above said first membrane to form a chamber between
said plurality of first membranes and said second membrane; a
pressure sensor responsive to a pressure in said chamber; drive
electronics responsive to said pressure sensor for controlling the
positions of each of said plurality of first membranes; and output
electronics responsive to the positions of said plurality of first
membranes.
18. The microphone of claim 17 wherein each of said plurality of
first membranes is substantially identical in size.
19. The microphone of claim 17 wherein each of said plurality of
first membranes is a multiple of a base sized membrane.
20. The microphone of claim 17 wherein said pressure sensor is one
of a capacitive sensor and a piezoresistive sensor.
21. The microphone of claim 17 additionally comprising a heater
carried by the substrate.
22. The microphone of claim 17 wherein said plurality of first
membranes and said second membrane are fabricated one above the
other to form an integral structure.
23. The microphone of claim 17 wherein said second membrane is
mechanically connected above said plurality of first membranes to
form a composite structure.
24. The microphone of claim 23 wherein said second membrane is one
of a fabricated membrane and a cover membrane.
25. The microphone of claim 17 wherein each membrane of said
plurality of first membranes is comprised of a first micro-machined
mesh and a first material sealing said mesh and wherein said second
membrane is comprised of a second micro-machined mesh and a second
material sealing said second mesh.
26. The microphone of claim 25 wherein said first material and said
second material are the same.
27. A microphone, comprising: a substrate; a plurality of first
micro-machined meshes supported by said substrate; a second
micro-machined mesh supported by the substrate and positioned above
said plurality of first meshes to form a chamber therebetween; a
material for sealing said plurality of first meshes to form a
plurality of first membranes and for sealing said second mesh to
form a second membrane, each of said plurality of first membranes
having first and second positions; a sensor responsive to said
chamber; drive electronics responsive to said sensor for
controlling the positions of each of said plurality of first
membranes; and output electronics responsive to the positions of
said plurality of first membranes for producing an output
signal.
28. The microphone of claim 27 wherein each of said plurality of
first membranes is substantially identical in size.
29. The microphone of claim 27 wherein each of said plurality of
first membranes is a multiple of a base sized membrane.
30. The microphone of claim 27 wherein said sensor is one of a
capacitive pressure sensor and a piezoresistive pressure
sensor.
31. The microphone of claim 27 additionally comprising a heater
carried by said substrate.
32. The microphone of claim 27 wherein said plurality of first
membranes and said second membrane are fabricated one above the
other to form an integral structure.
33. The microphone of claim 27 wherein the gaps of said second mesh
are larger than the gaps of said plurality of first meshes.
34. The microphone of claim 27 wherein said second membrane is
mechanically connected above said first membrane to form a
composite structure.
35. A method, comprising: fabricating a first micro-machined mesh
on a substrate; sealing said mesh to form a membrane; and
positioning a second membrane above said first membrane.
36. The method of claim 35 wherein said positioning includes
mechanically attaching one of a fabricated membrane and cover
membrane above said first membrane.
37. A method of fabricating stacked membranes, comprising: stacking
alternating layers of at least two different materials on a
substrate, certain of said layers being patterned; using a top
layer as an etch mask to form an upper mesh; removing said top
layer to expose a new top layer; using said new top layer to
protect said upper mesh while said upper mesh is released from said
substrate; removing said new top layer; using said upper mesh as an
etch mask to form and release a lower mesh from said substrate; and
depositing a sealant for sealing said upper and lower meshes.
38. The method of claim 37 wherein said stacking includes forming
alternating layers of metal and oxide, and wherein said top layer
is a layer of metal.
39. The method of claim 38 wherein a first of said layers of metal
is patterned to form said lower mesh, a second of said layers of
metal is patterned to define a chamber, and a third of said layers
of metal is patterned to form said upper mesh.
40. The method of claim 39 wherein said upper mesh has gaps of a
larger size than the gaps of said lower mesh, and wherein said
depositing step includes first sealing said lower mesh and then
sealing said upper mesh.
41. A method of fabricating stacked layers, comprising: forming a
first layer of a first material; forming a first layer of a second
material; patterning said first layer of said second material to
form a lower mesh; forming a second layer of said first material;
forming a second layer of said second material; patterning said
second layer of said second material to define a chamber above said
lower mesh; forming a third layer of said first material; forming a
third layer of said second material; patterning said third layer of
said second material to form an upper mesh above said chamber;
forming a fourth layer of said first material; forming a fourth
layer of said second material; and patterning said fourth layer of
said second material to act as an etch mask for forming said upper
mesh.
42. The method of claim 41 wherein said first material is an oxide
and said second material is a metal.
43. A method, comprising: forming an upper micro-machined mesh on a
substrate; releasing said upper mesh; forming and releasing a lower
mesh under said upper mesh; and sealing said lower and upper meshes
to form first and second membranes, respectively.
44. The method of claim 43 wherein said sealing includes deposition
of a polymer.
45. A method, comprising: sensing a pressure between an upper
membrane and a plurality of lower membranes, each of said plurality
of lower membranes having a micro-machined mesh; controlling the
position of each of said plurality of lower membranes in response
to said sensing; and monitoring the positions of said plurality of
membranes to provide an output signal.
46. The method of claim 45 wherein each of said plurality of lower
membranes has first and second positions, and wherein said
monitoring determines the position of each of said plurality of
lower membranes.
47. The method of claim 45 wherein said sensing includes sensing
pressure changes and wherein said controlling compensates for
sensed pressure changes.
48. A method of converting sound waves to a digital signal,
comprising: sensing a pressure in a chamber formed of an upper
membrane and a plurality of lower membranes, each of said lower
membranes having first and second positions, each of said lower
membranes having a micro-machined mesh; controlling, in response to
said sensing, whether each of said plurality of lower membranes is
in its first or second position; and outputting a digital signal
responsive to the positions of each of said plurality of lower
membranes.
49. The method of claim 48 wherein said plurality of lower
membranes are arranged in groups, each group being responsive to
produce one bit of the output digital signal.
50. The method of claim 48 wherein said plurality of lower
membranes are of various sizes, each size being responsive to
produce one bit of the output digital signal.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention is directed to micro-electromechanical
system (MEMS) devices and, more particularly, to MEMS devices which
include a flexible membrane.
[0003] 2. Description of the Background
[0004] Currently, acoustic devices such as microphones are
geometrically symmetrical with little internal structure. They
often consist of a rectangular or circular plate, or diaphragm,
whose motions are detected capacitively, piezoelectrically, or
piezoresistively to produce an analog signal. One difficulty in
microphone design is in providing a device having a sufficiently
linear response and required sensitivity over the frequencies of
operation.
[0005] It is desirable to convert the analog signal produced by a
microphone to a digital signal for further processing or storage.
However, analog-to-digital converters raise the cost of a product
because of the cost of designing and building such special and
precision electronic units. For example, the current source for the
most significant bit (MSB) of a multi-bit, digital representation
of an analog signal must have an error of less than the least
significant bit (LSB), which is usually several orders of magnitude
smaller than the MSB. A 14-bit analog to digital converter requires
the resolution and accuracy at 1/16384. The need exists for a
microphone that overcomes linearity problems and the cost and
difficulties associated with converting an analog signal to a
digital signal.
SUMMARY OF THE INVENTION
[0006] The present invention is a direct digital microphone which
provides a digital output signal directly in response to a
soundwave without the need to first produce an analog signal, and
then convert the analog signal to a digital signal. The direct
digital microphone is based on a structure, carried on a substrate,
which is comprised of a plurality of first, lower membranes and a
second, upper membrane. The lower membranes are each formed of a
micromachined mesh and are each sealed with a sealing material. The
upper membrane may be provided in a number of ways. First, the
upper membrane may be comprised of a micromachined mesh which is
fabricated and sealed above the lower membranes to form an integral
structure. In another embodiment, the upper membrane is formed of a
micro-machined mesh which is sealed to form a membrane. The
membrane is then mechanically connected above the lower membranes
to form a composite structure. In yet another embodiment, a cover
membrane of a suitable material is mechanically connected above the
lower membranes to form a composite structure.
[0007] The present invention is also directed to methods of
fabricating a stacked structure comprising at least two membranes.
The method of fabrication will vary depending upon whether an
integral or composite structure is being fabricated. The method may
be comprised of the steps of fabricating a first micro-machined
mesh on a substrate, sealing the mesh to form a membrane, and
positioning a second membrane above the first membrane. The
positioning may include mechanically attaching one of a fabricated
membrane and a cover membrane above the first membrane.
[0008] Another method of fabricating a stacked structure comprising
at least two membranes comprises the steps of stacking alternating
layers of at least two different materials on a substrate, with
certain of the layers being patterned. A top layer is used as an
etch mask to form an upper mesh. After the upper mesh is formed,
the top layer is removed to expose a new top layer. The new top
layer is used to protect the upper mesh while the upper mesh is
released from the substrate. The new top layer is then removed. The
upper mesh is used as an etch mask to form and release a lower mesh
from the substrate. A sealant is then deposited for sealing the
lower meshes and the upper mesh.
[0009] The present invention is also directed to a method of
fabricating stacked layers comprising forming a first layer of a
first material, e.g. oxide, and a first layer of a second material,
e.g. metal. The first layer of the second material is patterned to
form a lower mesh. A second layer of the first material and a
second layer of the second material are then formed. The second
layer of the second material is patterned to define a chamber above
the lower mesh. A third layer of the first material and a third
layer of the second material are formed. The third layer of the
second material is patterned to form an upper mesh above the
chamber. A fourth layer of the first material and a fourth material
of the second material are formed. The fourth layer of the second
material is patterned to act as an etch mask for forming the upper
mesh. The foregoing method will likely be performed by a CMOS
foundry to provide a structure having a number of stacked layers.
The structure may then be processed according to a post-processing
fabrication process to produce a structure having stacked
membranes. The post-processing fabrication process may include the
steps of: forming an upper micro-machined mesh on a substrate;
releasing the upper mesh; forming and releasing a lower mesh under
said upper mesh; and sealing the upper and lower meshes to form
first and second membranes, respectively.
[0010] As mentioned, the structure of the present invention may be
used to construct, for example, a microphone. A microphone
constructed of such a device is comprised of a plurality of first
membranes each formed by a micro-machined mesh supported by a
substrate. Each of the membranes has a first (up) and a second
(down) position. A second membrane is supported by the substrate
and positioned above the first membranes to form a chamber between
the plurality of first membranes and the second membrane. A
pressure sensor is responsive to pressure in the chamber. Drive
electronics are responsive to the pressure sensor for controlling
the positions of each of the plurality of first membranes. Output
electronics are responsive to the positions of the plurality of
first membranes to produce a digital output signal.
[0011] A method of converting soundwaves directly to a digital
signal is also disclosed. The method is comprised of sensing a
pressure in a chamber formed of an upper membrane and a plurality
of lower membranes. Each of the lower membranes has first and
second positions and each of the lower membranes is constructed of
a micro-machined mesh. The method also comprises controlling, in
response to the sensing, whether each of the plurality of lower
membranes is in its first or its second position. A digital signal
responsive to the positions of each of the plurality of lower
membranes is output.
[0012] The present invention provides a substantial advance over
the prior art in that it provides for the direct conversion of a
soundwave into a digital signal thereby eliminating the steps of
producing an analog signal and converting the analog signal to a
digital signal. The microphone may be built around a stacked
membrane structure having a chamber between the membranes. By
measuring the pressure in the chamber, and using the positions of
the plurality of lower membranes to maintain the pressure in the
chamber constant, problems associated with maintaining linearity of
the pressure measurement over a large range are eliminated. The
microphone can be constructed to be extremely sensitive around the
ambient (or starting) pressure because the control of the positions
of the lower membranes keeps the pressure very close to the ambient
(or starting) pressure. Furthermore, because the microphone may be
built using CMOS design techniques, advances in CMOS design can be
directly incorporated into the construction of the direct digital
microphone of the present invention. Extremely small and precise
microphones can be fabricated which can be employed in a variety of
electronic devices such as hearing aids, cell phones, and others.
Those advantages and benefits, and others, will be apparent from
the Description of the Preferred Embodiments herein below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] For the present invention to be easily understood and
readily practiced, the present invention will now be described, for
purposes of illustration and not limitation, in conjunction with
the following figures, wherein:
[0014] FIG. 1 is a block diagram of a digital microphone
constructed according to the teachings of the present
invention;
[0015] FIG. 2 is a diagram representing a top view looking down
onto a digital microphone of the type represented by the block
diagram of FIG. 1;
[0016] FIG. 3 is a cross-sectional view taken along the lines
III-III in FIG. 2;
[0017] FIG. 4 illustrates a substrate after a first layer of a
first material, e.g. oxide, and a first layer of a second material,
e.g. metal, have been formed;
[0018] FIG. 5 illustrates the substrate of FIG. 4 after the first
layer of the second material, e.g. metal, has been patterned to
form two lower meshes;
[0019] FIG. 6 illustrates the substrate of FIG. 5 after a second
layer of the first material, e.g. oxide, and a second layer of the
second material, e.g. metal, have been formed, and the second layer
of the second material has been patterned to define a chamber above
the two lower meshes;
[0020] FIG. 7 illustrates the substrate of FIG. 6 after a third
layer of the first material, e.g. oxide, and a third layer of the
second material, e.g. metal, have been formed, and the third layer
of the second layer has been patterned to form an upper mesh;
[0021] FIG. 8 illustrates the substrate of FIG. 7 after a fourth
layer of the first material, e.g. oxide, and a fourth layer of the
second material, e.g. metal, have been formed, and the fourth layer
of second material has been patterned to function as an etch mask
for the upper mesh;
[0022] FIG. 9 illustrates the substrate of FIG. 8 after another
layer of the first material, e.g. oxide, has been formed;
[0023] FIG. 10 illustrates the substrate of FIG. 9 after the first
post-processing step has been performed and the top layer of the
second material, e.g. metal, is used as an etch mask for the first
material, e.g. oxide;
[0024] FIG. 11 illustrates the substrate of FIG. 10 after an etch
of the second material, e.g. metal, has been performed and a
chamber formed under the upper mesh to release the upper mesh;
[0025] FIGS. 12A and 12B illustrate the substrate of FIG. 11 after
an etch of the first material, e.g. oxide, which forms and releases
the lower meshes;
[0026] FIGS. 13A and 13B illustrate the sealing of the upper and
lower meshes; and
[0027] FIG. 14 is a schematic of a four-bit, direct, digital
microphone constructed according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] A direct digital microphone 10 constructed according to the
teachings of the present invention will now be described in
conjunction with FIGS. 1, 2 and 3. FIG. 1 is a block diagram of the
direct digital microphone 10. FIG. 2 is a representation of a top
view looking down onto a direct digital microphone 10 of the type
represented by the block diagram of FIG. 1 while FIG. 3 is a
cross-sectional view taken along the lines III-III in FIG. 2. The
microphone 10 is comprised of an upper membrane 12, seen in FIGS. 1
and 3. The upper membrane 12 is not shown in FIG. 2. Positioned
beneath the upper membrane 12 is a plurality or array 14 of
individual lower membranes 16. The lower membranes 16 are sometimes
referred to herein as first membranes while the upper membrane 12
is sometimes referred to herein as the second membrane. A chamber
18, seen best in FIG. 3, is formed between the lower membranes 16
and upper membrane 12. The array 14 of lower membranes 16 and the
upper membrane 12 are all supported by a substrate 20. The
substrate 20 also carries a pressure sensor 22 and electronics 24.
The electronics 24 may include drive electronics and polling (or
output) electronics as will be described.
[0029] As a soundwave impinges on the upper membrane 12, the upper
membrane 12 deflects proportionally to the sound pressure causing a
change in volume of chamber 18. The change in volume happens
quickly compared to the time needed for heat flow, such that an
adiabatic compression or expansion takes place, changing the
pressure of the air within the chamber 18. The pressure sensor 22
senses if the pressure of the air in the chamber 18 deviates from
atmospheric (or starting) pressure. Drive electronics 24 are
responsive to the pressure sensor 22.
[0030] Each of the lower membranes 16 has first and second
positions. A first position, for example an equilibrium position,
may be representative of a logic "0" while a second position, which
may be achieved through the application of, for example, a charge
differential between the membrane and the substrate, may be
representative of a logic "1". The drive electronics, responsive to
the sensor 22, will control the position of the individual
membranes 16 to maintain a constant pressure in chamber 18. Thus,
the sensor 22, drive electronics 24, and array 14 of individual
membranes 16 may be thought of as a negative feedback loop. Because
each of the individual membranes 16 is in either one of its two
states, the pressure/volume correction the individual membranes
implement is proportional to the number that are deflected (i.e. in
their second state). Polling electronics keep track of the number
of membranes that are deflected at any time, for example, by
determining how many are activated by their associated drive
electronics. The number of individual membranes 16 that are
deflected may be directly output as a digital signal representative
of the pressure sensed by the upper membrane 12.
[0031] The pressure sensor 22 may be a capacitive or piezoresistive
sensor comprised of a membrane similar to membranes 16, but larger.
Because the pressure sensor 22 only has to measure whether the
pressure is above or below the equilibrium or starting pressure,
there is no requirement for linearity. Instead, high sensitivity
around the equilibrium pressure is a main design concern. A
polysilicon heater (not shown), or other type of heater, may be
integrated inside or near the pressure sensor 22 to set the
mechanical operating point and maximize sensitivity.
[0032] The array 14 of individual membranes 16 may be a uniformly
distributed n x n array. Each membrane 16 snaps independently and
in a predetermined sequence between its first at "rest" (up)
position and its second "deflected" (down) position. The sensed
sound pressure is measured in n.sup.2 levels and thus digitized
electromechanically. In another embodiment, the array 14 may be
divided into groups. Each group has a certain number of membranes
16 assigned to it that snap simultaneously as a group. The number
of membranes 16 in each group represents the weight significance of
that group. For example, a group with one membrane corresponds to
the least significant bit, a group with two membranes corresponds
to the next least significant bit, and so forth. If an 8-bit direct
digital microphone is needed, the membranes may be divided into
eight groups with each group having 1, 2, 4, 8, 16, 32, 64 and 128
individual membranes, respectively. In yet another embodiment, the
size of the individual membranes 16 may be varied such that the
next least significant bit is twice the area of the least
significant bit, the next bit has four times the area of the least
significant bit, the next bit has eight times the area of the least
significant bit, etc. The array 14 may be comprised of hundreds or
thousands of individual membranes 16.
[0033] The electronics 24 may include, but are not limited to,
preamplifiers, operational amplifiers, charge pumps, select
circuits, etc. The electronics 24 may include an interface between
the sensor 22 and the drive electronics which control the positions
of the individual membranes 16. The construction and operation of
the electronics 24 is considered conventional and not further
described herein.
[0034] The direct digital microphone 10 of the present invention is
constructed around a stacked structure, i.e. an upper membrane 12
positioned above one or more lower membranes 16. At least the lower
membranes 16 are comprised of a micro-machined mesh which is
sealed. Construction of such a sealed mesh is known. See, for
example, International Publication No. WO 01/20948 A2, published
Mar. 22, 2001 which is hereby incorporated by reference. We turn
next to a discussion of how to fabricate such a stacked
structure.
[0035] In FIG. 4, the substrate 20 is shown after having been
subjected to a CMOS fabrication process to produce electronics 24.
The fabrication of the electronics 24 does not form a feature of
the present invention and therefore is not discussed. Upon
substrate 20, a first layer 26 of a first material, e.g oxide, is
formed. The first layer 26 of the first material, e.g. oxide, may
be formed in any conventional manner such as, for example,
deposition or, in the case of oxide, thermally oxidizing substrate
20. Thereafter a first layer 28 of a second material, e.g. metal
(in this case aluminum), is formed using any appropriate technique.
The layer 28 of the second material is then patterned using any
known techniques such as applying photoresist, curing the
photoresist according to a mask, and then removing portions of the
layer 28 of the second material resulting in the patterned layer
28' shown in FIG. 5. The layer 28 may be patterned in the area of
electronics 24 so as to interconnect components to provide the
desired functions. More importantly, the patterned layer 28'
provides a mesh 30 and a mesh 32 of the type disclosed, for
example, in WO 01/20948 and which will be released and formed into
membranes according to the process described below.
[0036] Turning now to FIG. 6, a second layer 34 of the first
material and a second layer 36 of the second material are formed on
substrate 20. The second layer 36 of the second material 36 is
patterned to have a portion 38 defining a chamber, the function of
which will be described herein below.
[0037] In FIG. 7, a third layer 40 of a first material and a third
layer 42 of the second material are formed. The third layer 42 of
the second material is patterned to form a mesh 44 of the type
disclosed in, for example, WO 01/20948. In FIG. 8, a fourth layer
46 of the first material and a fourth layer 48 of the second
material are formed. The fourth layer 48 of the second material is
patterned so as to provide an etch mask for forming the upper mesh
44 as will be described below. To enable the layer 48 of the second
material to function as an etch mask, the width of the beams
forming the mesh 44 may be approximately 0.2 .mu.m less than the
width of the beams forming the pattern formed in the layer 48 of
the second material. Thereafter, as shown in FIG. 9, another layer
50 of the first material may be formed.
[0038] It is anticipated that the stacked structure 52 shown in
FIG. 9 will be produced by a CMOS foundry. Thereafter, the stacked
structure 52 will be subjected to post-processing steps, described
in conjunction with FIGS. 10-13B, to fabricate an integral stacked
membrane structure. Although the stacked structure 52 has been
described in conjunction with CMOS processing and alternate layers
of a first material, e.g. oxide, and a second material, e.g. metal,
other processing techniques and other materials may be used while
remaining within the scope of the present invention. One criterion
for selecting materials is to select materials which are responsive
to different etching processes, so that one material may be used as
a mask, while the other material is being etched, as will become
more apparent from the description of the post-processing steps
illustrated in FIGS. 10-13B.
[0039] In FIG. 10, the top layer 50 of the first material, e.g.
oxide, has been removed using an etch selective to the first
material, e.g. an oxide etch. The first material etch also etches
through the layers 46, 40 of the first material using the new top
layer, the fourth layer 48 of the second material as an etch mask.
Any directional first material etching technique may be used. When
the first material etch etches through the fourth layer 46 of the
first material, the upper mesh 44 is formed, but is encased in the
first material. Etching stops upon reaching the layer 36 of the
second material.
[0040] In FIG. 11, it is seen that the new top layer, i.e. the
fourth layer 48 of the second material has been removed to expose
yet another new top layer, fourth layer 46 of the first material.
The fourth layer 46 of the first material is used to protect the
mesh 44 while allowing the portion 38 to be removed through an
isotropic etch of the second material. With portion 38 removed as
seen in FIG. 11, the upper mesh 44 is released from the substrate.
Because the mesh 44 is imbedded in the first material, e.g. oxide,
on both its faces forming a symmetric stress distribution, buckling
is expected to be minimal.
[0041] The new top layer 46 of the first material is stripped as
shown in FIG. 12A using an etch selective to the first material,
e.g. oxide, which also etches through the layers 34, 26 of the
first material. As shown in FIG. 12A, when the layer 26 of the
first material is etched, the individual meshes 30, 32 are formed.
Continuation of the etching step, or as a result of a separate
step, the silicon substrate is undercut as illustrated in FIG. 12B
thereby releasing meshes 30, 32. The separation between lower
meshes 30, 32 and upper mesh 44 may be approximately 1 to 3
.mu.m.
[0042] After the lower meshes 30, 32 have been released, a polymer
deposition step is performed which first seals the lower meshes as
seen in FIG. 13A to form membranes 60, 62, which are two examples
of the membranes 16 of FIG. 2, and then to form membrane 64, which
performs the function of the upper membrane 12 illustrated in FIGS.
1 and 3. To insure that the lower meshes 30, 32 are sealed before
the upper mesh 44, the upper mesh may be constructed to have gaps
between the beams which are larger than the gaps between the beams
forming lower meshes 30, 32. For example, the gaps between the
beams forming the upper mesh 44 may be approximately 1.5 .mu.m
while the gaps between the beams forming the lower meshes 30, 32
may be approximately 0.5 .mu.m. Accordingly, the beams forming
lower meshes 30, 32 may be wider than the beams forming upper mesh
44.
[0043] A similar technique may be used with a three-metal CMOS
process to create meshes in metal 2, very close to the substrate,
even though metal 3 would be sacrificed in such a process.
[0044] Those of ordinary skill in the art will recognize that the
pattern used to form the lower meshes may also be used to form
traces 70, shown in FIG. 14, for interconnecting the lower meshes
to drivers. As shown in FIG. 14, lower meshes 71-74 are connected
to bit 3 driver 75, three of lower meshes 76-79 are connected to
bit 2 driver 80, two of lower meshes 81-84 are connected to bit 1
driver 85, and one of lower meshes 86-89 are connected to bit 0
driver 90. A substrate bias 91 may be used to bias the substrate
at, for example, a negative voltage. Through the application of a
positive voltage via bit drivers 75, 80, 85, 90 to the lower
meshes, the position of the membranes can be controlled.
[0045] In another embodiment of the present invention, the CMOS
process is used to build up stacks of alternating materials
sufficient to form micro-machined meshes which may then be sealed
by a sealing material. In one area of the substrate, the plurality
of lower meshes is formed, and in another area of the substrate the
upper mesh is formed. After sealing of all of the meshes, the
substrate is cut, and the upper membrane is mechanically bonded in
a face-to-face fashion so that it is positioned over the plurality
of lower membranes to form a composite device. Such a bonding
technique could be used in both a chip-to-chip fashion or at the
wafer level.
[0046] In yet another embodiment in which a composite device is
produced, a cover membrane is a passive component which is simply
glued or otherwise mechanically attached above the array of lower
membranes. The cover membrane may be a thin layer of plastic or
polymer mechanically attached to a chip having formed thereon the
array 14 of lower membranes 16, the pressure sensor 22 and
electronics 24. Of course, the integrated approach discussed above
eliminates problems accompanying mechanically attaching the upper
membrane such as sealing and alignment.
[0047] While the present invention has been described in
conjunction with preferred embodiments thereof, those of ordinary
skill in the art will recognize that many modifications and
variations may be implemented while still falling within the scope
of the present invention. For example, processing techniques other
than CMOS techniques may be utilized. Other types of materials and
process steps may be substituted for those described in the
preferred embodiment while remaining within the scope of the
present invention. The description of presently preferred
embodiments is not intended to limit the scope of the present
invention, which is defined by the following claims.
* * * * *